Pointers in C
Pointers in C
Xiao Jia
Shanghai Jiao Tong University
November 22, 2011
Pointers in C
Outline
Outline
Overview of pointers
Pointers in depth
Q & A
* If you can’t read this, move closer!
Pointers in C
Overview of pointers
Overview of pointers
Definition & declaration
Assignment & dereferencing
Arrays
Pointer arithmetic
Indexing
Structures and unions
Multiple indirection
const
Function pointers
Pointers in C
Overview of pointers
Definition & declaration
Definition
A pointer is a memory address.
int foo;
int *foo_ptr = &foo;
Pointers in C
Overview of pointers
Definition & declaration
Declaration
int* ptr_a, ptr_b;
int *ptr_a;
int ptr_b;
int *ptr_a, ptr_b;
int ptr_b, *ptr_a;
Pointers in C
Overview of pointers
Definition & declaration
Declaration
int *ptr_a, *ptr_b;
int ((not_a_pointer)), (*ptr_a), (((*ptr_b)));
This is useful for declaring function pointers (described later).
Pointers in C
Overview of pointers
Assignment & dereferencing
Assignment & dereferencing
int foo;
int *foo_ptr = &foo;
foo_ptr = 42;
int bar = *foo_ptr;
*foo_ptr = 42; //Sets foo to 42
Pointers in C
Overview of pointers
Arrays
Arrays
int array[] = { 45, 67, 89 };
The variable array is an extra-big box: three ints’ worth of
storage.
array == &array == &array[0]
array
pointer to array
pointer to the first element of array
Pointers in C
Overview of pointers
Pointer arithmetic
Pointer arithmetic
int *array_ptr = array;
printf(" first element: %d\n", *(array_ptr++));
printf("second element: %d\n", *(array_ptr++));
printf(" third element: %d\n", *array_ptr);
first element: 45
second element: 67
third element: 89
int pointers are incremented or decremented by sizeof(int)
bytes. void pointers are incremented or decremented by 1 byte
since sizeof(void) is illegal.
Pointers in C
Overview of pointers
Indexing
Indexing
int array[] = { 45, 67, 89 };
int *array_ptr = &array[1];
printf("%d\n", array[0]); // 45
printf("%d\n", array_ptr[1]); // 89
Pointers in C
Overview of pointers
Structures and unions
Structures and unions
struct foo {
size_t size;
char name[64];
int answer_to_ultimate_question;
unsigned shoe_size;
};
struct foo my_foo;
my_foo.size = sizeof(struct foo);
Pointers in C
Overview of pointers
Structures and unions
Structures and unions
struct foo *foo_ptr = &my_foo;
(*foo_ptr).size = new_size;
foo_ptr->size = new_size;
struct foo **foo_ptr_ptr = &foo_ptr;
(*foo_ptr_ptr)->size = new_size;
(**foo_ptr_ptr).size = new_size;
Pointers in C
Overview of pointers
Multiple indirection
Multiple indirection
int a = 3;
int *b = &a;
int **c = &b;
int ***d = &c;
*d == c
**d == *c == b
***d == **c == *b == a == 3
Pointers in C
Overview of pointers
const
const
const int *ptr_a;
int const *ptr_a;
int const *ptr_a;
// int is const; cannot do *ptr_a = 42
int *const ptr_b;
// can change *ptr_b; cannot do ptr_b++
Pointers in C
Overview of pointers
Function pointers
Function pointers
Consider strcpy.
enum { str_length = 18U };
char src[str_length] = "This is a string.",
dst[str_length];
strcpy(dst, src);
Pointers in C
Overview of pointers
Function pointers
Declaring function pointers
char *strcpy(char *dst, const char *src);
// Just for reference
char *(*strcpy_ptr)(char *dst, const char *src);
// Pointer to strcpy-like function
strcpy_ptr = strcpy;
strcpy_ptr = &strcpy;
//strcpy_ptr = &strcpy[0];
Pointers in C
Overview of pointers
Function pointers
Parameter names are optional
char *(*strcpy_ptr_noparams)(char *, const char *) =
strcpy_ptr;
strcpy_ptr =
(char *(*)(char *, const char *))my_strcpy;
char *(**strcpy_ptr_ptr)(char *, const char *) =
&strcpy_ptr;
Pointers in C
Overview of pointers
Function pointers
Array of function-pointers
char *(*strcpies[])(char *, const char *) =
{ strcpy, strcpy, strcpy };
strcpies[0](dst, src);
Pointers in C
Overview of pointers
Function pointers
Declaring exercise
Declare the following in a single line:
a function f with no parameters returning an int
a function fip with no parameter specification returning a
pointer to an int
a pointer pfi to a function with no parameter specification
returning an int
(taken from C99 standard)
Pointers in C
Overview of pointers
Function pointers
Declaring exercise
Declare the following in a single line:
a function f with no parameters returning an int
a function fip with no parameter specification returning a
pointer to an int
a pointer pfi to a function with no parameter specification
returning an int
(taken from C99 standard)
int f(void), *fip(), (*pfi)();
Pointers in C
Overview of pointers
Function pointers
Function returning a function pointer
char *(*get_strcpy_ptr(void))(char *dst,
const char *src);
strcpy_ptr = get_strcpy_ptr();
Pointers in C
Overview of pointers
Function pointers
typedef
typedef char *(*strcpy_funcptr)(char *, const char *);
strcpy_funcptr strcpy_ptr = strcpy;
strcpy_funcptr get_strcpy_ptr(void);
Pointers in C
Overview of pointers
Function pointers
Summary
Declaring
void (*foo)(int);
Initializing
void foo();
func_ptr = foo;
func_ptr = &foo;
Invoking
func_ptr(arg1, arg2);
(*func_ptr)(arg1, arg2);
Pointers in C
Pointers in depth
Pointers in depth
What is a pointer?
Pointer types and arrays
Pointers and strings
Pointers and structures
Multi-dimensional arrays
Dynamic allocation of memory
When to use pointers?
Pointers in C
Pointers in depth
What is a pointer?
What is a pointer?
int j, k, *ptr;
k = 2;
j = 7;
k = j;
ptr = &k;
*ptr = 7;
What is a variable?
What is an address?
What is an object?
Pointers in C
Pointers in depth
What is a pointer?
What is a pointer?
int j, k;
k = 2;
j = 7;
k = j;
What is lvalue?
What is rvalue?
Pointers in C
Pointers in depth
What is a pointer?
Object & lvalue
int j, k;
k = 2;
j = 7;
k = j;
An object is a named region of storage
An lvalue is an expression referring to an object
Pointers in C
Pointers in depth
Pointer types and arrays
Pointer types
int *ptr;
char *str;
double *dptr;
What is the size of a pointer?
Pointers in C
Pointers in depth
Pointer types and arrays
Pointer types
int *ptr;
*ptr = 2;
What is the problem with the code above?
Pointers in C
Pointers in depth
Pointer types and arrays
Pointer types
int *ptr, k;
ptr = &k; // What is the value of ptr?
*ptr = 10; // What is the value of k?
ptr++;
*ptr = 11;
What is the problem with the code above?
Pointers in C
Pointers in depth
Pointer types and arrays
Pointers and arrays
int my_array[] = {1, 23, 17, 4, -5, 100};
int *ptr;
ptr = &my_array[0];
ptr = my_array;
my_array = ptr; // It’s a named region of storage!
What is the problem with the code above?
What is the difference between ptr and my array?
Pointers in C
Pointers in depth
Pointers and strings
Pointers and strings
char my_string[40];
my_string[0] = ’A’;
my_string[1] = ’c’;
my_string[2] = ’m’;
my_string[3] = ’\0’;
char my_string[40] = {’A’, ’c’, ’m’, ’\0’};
char my_string[40] = "Acm";
char *my_string = "Acm";
const char *my_string = "Acm";
Pointers in C
Pointers in depth
Pointers and strings
Implementing strcpy
char *my_strcpy(char dest[], char src[]) {
int i = 0;
while (src[i] != ’\0’) {
dest[i] = src[i];
i++;
}
dest[i] = ’\0’;
return dest;
}
Pointers in C
Pointers in depth
Pointers and structures
Pointers and structures
struct Man { int age; };
struct Superman { Man man_part; int power; };
void print_man(void *p) {
cout << "Age: " << ((Man *)p)->age << endl;
}
void print_superman(void *p) {
print_man(p);
cout << "Power: " << ((Superman *)p)->power << endl;
}
Pointers in C
Pointers in depth
Pointers and structures
Pointers and structures
struct Man a = { 25 };
struct Superman b = { a, 250 };
print_man(&a); // Age: 25
print_superman(&b); // Age: 25
// Power: 250
b.man_part.age++;
print_man(&a); // Age: 25
print_superman(&b); // Age: 26
// Power: 250
Pointers in C
Pointers in depth
Pointers and structures
Arrays of length zero
struct line {
int length;
char contents[0];
};
struct line *this_line = (struct line *)
malloc( sizeof(struct line) + this_length );
this_line->length = this_length;
strcpy(this_line->contents, this_contents);
Pointers in C
Pointers in depth
Pointers and structures
Arrays of length zero
struct foo { int x; int y[]; };
struct bar { struct foo z; };
struct foo a = { 1, {2, 3, 4} }; // Valid.
struct bar b = { { 1, {2, 3, 4} } }; // Invalid.
struct bar c = { { 1, {} } }; // Valid.
struct foo d[1] = { { 1, {2, 3, 4} } }; // Invalid.
Pointers in C
Pointers in depth
Multi-dimensional arrays
Multi-dimensional arrays
int multi[5][10];
multi[row][col]
*(*(multi + row) + col)
// *(multi + row) -> X
// *(X + col)
&multi == 100
sizeof(int) == 4
&multi[3][5] == ???
Pointers in C
Pointers in depth
Dynamic allocation of memory
Allocate & release an int
int *p = (int *) malloc(sizeof int);
*p = 100;
free(p);
int *p = new int;
*p = 100;
delete p;
Pointers in C
Pointers in depth
Dynamic allocation of memory
Allocate & release a 1-dimension array
int *a, i;
a = (int *) malloc(10 * sizeof(int));
for (i = 0; i < 10; i++) {
a[i] = i;
}
free(a);
int *a = new int[10];
for (int i = 0; i < 10; i++) {
a[i] = i;
}
delete[] a;
Pointers in C
Pointers in depth
Dynamic allocation of memory
Allocate a 2-dimension array
int **a = new int*[10];
for (int i = 0; i < 10; i++) {
a[i] = new int[20];
for (int j = 0; j < 20; j++) {
a[i][j] = i + j;
}
}
Pointers in C
Pointers in depth
Dynamic allocation of memory
Release a 2-dimension array
for (int i = 0; i < 10; i++) {
delete[] a[i];
}
delete[] a;
Pointers in C
Pointers in depth
Dynamic allocation of memory
Allocate a 3-dimension array
int ***a = new int**[10];
for (int i = 0; i < 10; i++) {
a[i] = new int*[20];
for (int j = 0; j < 20; j++) {
a[i][j] = new int[30];
for (int k = 0; k < 30; k++) {
a[i][j][k] = i + j + k;
}
}
}
Pointers in C
Pointers in depth
Dynamic allocation of memory
Allocate a fluctuated 2-dimension array
int **a = new int*[10];
for (int i = 0; i < 10; i++) {
a[i] = new int[i + 1];
for (int j = 0; j <= i; j++) {
a[i][j] = i + j;
}
}
Pointers in C
Pointers in depth
When to use pointers?
When to use pointers?
Indirect addressing
Dynamic (run-time) addressing
Polymorphism
Pointers vs. references
Pointers may be NULL
References have to be valid (but may not if misused)
As parameters, small objects should behave like ints, e.g.
std::string.
Resource management
Must NOT have memory leaks
Acquiring and releasing tend to behave in a well-nested fashion
Across the borders of functions/methods, use smart pointers
Pointers in C
Q & A
Q & A
Thank you for listening!
Any questions?
Pointers in C
Q & A
References
Ted Jensen. A Tutorial on Pointers and Arrays in C. Sept.,
2003.
Peter Hosey. Everything you need to know about pointers in
C. Jan. 16, 2010.
Cprogramming.com. Function Pointers in C and C++.